A magnetic resonance imaging (MRI) apparatus is an imaging apparatus which excites nuclear spins of a patient placed in a static magnetic field with a radio frequency (RF) signal at Larmor frequency and thereby generates an image by reconstructing a magnetic resonance signal generated from the patient as a result of the excitation.
As the MRI apparatus applies the radio-frequency signal (RF signal) to the patient to obtain the magnetic resonance signal, temperature of the patient is raised. Thus, in relation to imaging by means of the MRI apparatus, from the standpoint of safety, a specific absorption ratio (SAR) has been defined as energy absorbed per unit mass of the patient and an upper limit of SAR, i.e., a safety standard value of SAR, has been prescribed as an IEC (International Electrotechnical Commission) standard (IEC 60601-2-33). More specifically, SAR (unit: W/kg) is defined as energy of the RF signal absorbed by 1 kg of living tissue, and upper limits of average SAR over arbitrary 10 seconds (hereinafter referred to simply as a 10-second average SAR) and average SAR over the most recent 6 minutes (hereinafter referred to simply as a 6-minute average SAR) have been prescribed for each imaging site such as the whole body or the head.
Since the upper limit of SAR has been prescribed for each body site as described above, a SAR calculation method for each body site has been provided.
Also, for examinations based on an MRI apparatus, there is an imaging method which acquires detailed diagnostic images by increasing contrast of a tumor or target site using a contrast medium. Such examinations using a contrast medium have a problem of difficulty to determine whether the contrast medium has reached a destination.
Thus, an MRI apparatus has been provided which carries out imaging at a desired destination of a contrast medium in advance before injection of the contrast medium, takes scans successively after injection of the contrast medium, and automatically starts imaging when signal strength at the destination of the contrast medium increases.
In relation to imaging by means of the MRI apparatus, in addition to the 10-second average SAR and 6-minute average SAR described above, an upper limit of long MR examination specific absorbed energy has been prescribed as well. The long MR examination specific absorbed energy is a sum total of SAR per examination, i.e., a cumulative value or integration value of SAR per examination. The IEC standard prescribes that when the upper limit of the long MR examination specific absorbed energy is exceeded, imaging must not be carried out any more. Hereinafter the long MR examination specific absorbed energy will be referred to simply as SAE (Specific Absorbed Energy).
In an examination using a contrast medium, when imaging (hereinafter referred to as monitoring imaging) is carried out by continuously applying an RF signal and displaying reconstructed images to conduct observation, unless values of SAR and SAE (long MR examination specific absorbed energy) are controlled properly based on the safety standard values, the examination has to be stopped halfway even if the contrast medium has not reached its destination. Furthermore, even if the contrast medium reaches its destination by means of the monitoring imaging, there remains a problem of being unable to carry out imaging (hereinafter referred to as main imaging) intended to collect images subsequently.
Also, recently, a treatment and examination method known as interventional MRI has been attracting attention because of low invasiveness, where the method involves inserting a catheter or puncture needle into a subject or performing a medical operation such as an endoscopic surgery while carrying out imaging. The interventional MRI involves carrying out imaging continuously using an MRI apparatus, as during verification of contrast medium injection, while performing treatment activities such as medical procedures (hereinafter, the sequential imaging which involves treatment activities will be referred to as interventional imaging). Therefore, unless rises in body temperature, i.e., increases in the SAR and SAE (long MR examination specific absorbed energy) described above, are controlled properly, there can arise a situation in which imaging has to be stopped halfway through a medical procedure.
Thus, there is demand for an MRI apparatus which automatically adjusts imaging conditions such that upper limits of SAR and SAE (long MR examination specific absorbed energy) values will not be exceeded when monitoring imaging or interventional imaging is carried out using the MRI apparatus.